In certain optical networks, such as passive optical networks, signals are sent along optical fibres which may, for example, include a 3 dB coupler at a node in the network so that the signals are transmitted over the entire network. In many cases, however, it would be desirable for optical signals to be routed along the network, for example in order to improve security of the system to unauthorised access to data. In electric networks, data routing typically involves reading the address of the data into a computer at a node of the network and determining the path of the data in accordance with the data address. Such a system has the disadvantage that the rate at which data can be sent along the network is limited by the speed at which the computer can perform the necessary reading and comparison operations. Even in optical networks that involve routing, the switching operates in the same manner as in electrical networks, with the result that the data rate is still limited by the computer speed. For example an optical switching matrix is described in EP-A-0 497 667 and by Jacob & Gabriagues, Communication et Transmission, Vol. 16, No. 2, 1994 pp. 5-14, in which incoming optical signals contain routing tags that are read and processed by a computer in order to assign carrier wavelengths to the signals that correspond to the signal destinations. The signals, modulated to different wavelengths corresponding to their destinations, are then sent to a space switching stage, and the signals leaving the space switching stage are filtered by optical bandpass filters in each output line to allow only those signals with the appropriate carrier wavelengths to pass.